Compression

Today I was wandering through harbor freight tools and found a compression tester for cheap. So i bought it to satisfy my curiosity but I don't Know what the compression is supposed to be for a stock 250ci. Could anyone let me know?

Thanks,
Brian

 

after 3 revolutions, i think it's supposed to be somewhere around 140 or so PSI

 

if it should be 140 then i now completely understand my low gas mileage and all the smoke out the tail pipe

 

So what is yours?

 

90-115 every cylinder is different

 

If they were all around 115 I wouldn't worry quite as much... but that 90 isn't great at all.

 

you need to do it when the motor is hot. then if you have a big difference on some cylinders put a cap full of oil in each cylinder and try it again. its pretty important that all cylinders be the same temp and that the battery has a full charge on it and is in good condition for the test to be accurate. if the cylinders that get tested towards the end are lower than the ones at the begining then odds are that the conditions of the test have changed.

 

That's a very good point.

You could test it in one direction and get numbers, then test it again in reverse order to make sure nothing has changed.

 

I dont know how much faith i would put into the harbor freight gauge being 100% accurate either.

 

Thanks I did test it hot it and once in both directions and got about the same results cylinders 1and 2 are both at about 90 the others are all somewhere around 115-120.

 

Now you need to determine whats bad on those 2 cyls.Burnt valves or bad rings...could also be a head gasket fire ring between the 2 bores going bad.Do you use/loose coolant???

 

Hey. my 250 cid compression are around 55-75 psi when cold (i don´t like to do the testing with the engine hot because of the spark plugs threads could break or something - yes, i know, it should be done when hot), i didn´t the oil part either, i know the engine is getting tired already. My car haves a sligt fail on idle, but doesn´t make any smoke. I make 88 miles a day @ 85 MPH and gives good mileage either.

Yes, i´m saving to overhaul the engine soon... hehehehehe

 

Victor...as long as you use a good screw in compression tester you shouldnt have any problems with yanking the threads out of the head.If i am dealing with a problem child I will do the comp test cold and hot to see how drastic a change I actually get.

 

Compression tests tell you very little. The spring on the check valve can make your compression read higher or lower. When doing one you need to have the engine at operating temp for the most accuracy and the throttle should be block at wide open. Perform the dry check first (it doesn't hurt to have a charger on the battery to keep it at top charge) then squirt about a tablespoon of oil in each cylinder and crank it over a few times to get the oil on the rings. Crank the engine the same number of times (watch the needle jump) on each cylinder (I use six times) and record the pressures as you go. The pressure is not as important as having all the pressures within 10% of the highest.
example 1:
160, 155, 155, 160, 145, 150 would be within factory tollerance. 160 x 10% =16 145 + 16 = 161 All are within 10% of the high reading.
example 2:
200, 195, 190, 175, 180, 190 wold be out of factory tollerance. 200 x 10% = 20 175 + 20 = 195 #4 cylinder is more than 10% less than 200.

Most of the 70 V-8s should have somewhere close to or more than 150 PSI when performed as I outlined above. (modified engines are anyone's bet - more or less depending on compression and cam overlap)

 

loosen the plugs when the motor is cool and retighten them to just past finger tight. then warm up the motor. you wont damage any threads this way. also cast iron heads are pretty hard to mess up the threads.

if you have a air compressor you can do a leak down test with the compression check gage. you will want to do this on the cylinders that read low on the compression check. just take out the valve core in the compression tester. then adapte it to the air hose. put the cylinder that your testing on top dead center with the rotor in the dizzy pointing at that cylinders post on the cap. hook up the air. if the motor turns when you hook up the air you dont have it rigth on top dead center. once you get it to stay on top dead center with the air hooked up you can see where its going. check the intake by opening the carb throttle and see if air is coiming out of there. if air is coming out you have a bad intake valve. then check the exhast pipe and see if air is coming out of the tail pipe. if air is coiming out there you have a bad exhast valve. if there is no air coming out then the rings are bad or you have a hole in the piston.